Cirque Headwall Research Articles

Ice-buttressing-controlled rock slope failure on a cirque headwall, Lake District, UK

Abstract. Rock slope failures in the Lake District, UK, have been associated with deglacial processes after the Last Glacial Maximum, but the controls and timing of the failures remain poorly known. A cirque headwall failure was investigated to determine failure mechanisms and timing. The translated wedge of rock is thin and lies on a steep failure plane, yet the friable strata were not disrupted by downslope movement. Fault lines and a failure surface, defining the wedge, were used as input to a numerical model of rock wedge stability. Various failure scenarios indicated that the slope was unstable and would have failed catastrophically if not supported by glacial ice in the base of the cirque. The amount of ice required to buttress the slope is insubstantial, indicating likely failure during the thinning of the cirque glacier. We propose that, as the ice thinned, the wedge was lowered slowly down the cirque headwall, gradually exposing the failure plane. A cosmogenic 10Be surface exposure age of 18.0±1.2 ka from the outer surface of the wedge indicates Late Devensian de-icing of the backwall of the cirque, with a second exposure age from the upper portion of the failure plane yielding 12.0±0.8 ka. The 18.0±1.2 ka date is consistent with a small buttressing ice mass being present in the cirque at the time of regional deglaciation. The exposure age of 12.0±0.8 ka represents a minimum age, as the highly fractured surface of the failure plane has experienced post-failure mass-wasting. Considering the chronology, it appears unlikely that the cirque was reoccupied by a substantial ice mass during the Younger Dryas stadial.

Rock glaciers in the Eastern Cascades, Washington State, USA: Impacts of selected variables on spatial distribution and landform dimensions

Recent regional-scale studies of rock glaciers have added much to our knowledge of rock glaciers and associated permafrost spatial distribution. In this study, we used Google Earth Pro imagery and selected field work to identify and classify 159 rock glaciers in the marine-influenced, continental margin, Eastern Cascades of Washington state in Western North America. Most rock glaciers are tongue-shaped, talus-derived, intact features. The majority are found in the Northeastern Cascades where temperatures are lower because of high elevations, high latitudes, and increasingly continental conditions. Most rock glaciers are also clustered around high peaks and ridges at the bases of cirque headwalls where steep, converging slopes provide ample debris. Dimensions of rock glaciers increase with increasing maximum elevation, area, relief, length, and slope of rocksheds. In comparison to other ranges, Eastern Cascades rock glaciers are generally small and low in spatial density, perhaps because of lower elevations and younger landscapes. Despite this, rock glaciers are significant components of the alpine/subalpine geomorphic continuum in the Eastern Cascades typically occupying late Pleistocene or Holocene cirques. Intact rock glaciers suggest that discontinuous permafrost currently exists down to ~1945 m elevation while relict features suggest that permafrost once extended down to ~1870 m elevation. As climate warms, slowly melting ice in rock glaciers will play a larger role in providing base flow for streams previously primarily supplied by snow and glacier melt.

The erosion of glaciated mountains: evidence from hypsoclinometry

Mountain glaciation involves the erosion of cirques and troughs, which increase steep slopes but also produce gentle slopes in cirque floors and trough floors. This is expected to increase the variability of slope gradients at related altitudes. Taking a whole mountain range, its distributions of altitude and slope can be analysed to establish a signal of glacial modification. Frequency distributions of altitude (hypsometry) and gradient (clinometry) alone do not seem adequate. Taking these two variables together – hypsoclinometry, plotting slope gradient against altitude – is more promising. Frequency distributions of slope gradient at different altitudes are exemplified here for mountain ranges in British Columbia and Romania, together with altitudinal variations of steep or gentle slopes. Cirque headwalls give the clearest morphometric signature of glaciation. Steep (especially the steepest) slopes are concentrated at cirque altitudes, increasing mean, median, standard deviation (SD) and inter-quartile range (IQR) of gradients, especially above cirque floors. There is only a small increase in SD and IQR at cirque floor altitudes. Hypsometric maxima and increased proportions of gentle slopes at cirque floor altitudes are clear only in mountain ranges densely occupied by cirques. This relates to the small proportion of each cirque (about 28%) occupied by the floor. Concentrations of steep slope aspects in directions favoured by local glaciers provide further evidence of glacial modification. The most general morphometric effect of glaciation, however, is the increase in steep slopes at cirque headwall altitudes. Thus it is possible to rank mountain ranges by degree of glacial modification.

Glacial erosion: status and outlook

AbstractGlacier-erosion rates range across orders of magnitude, and much of this variation cannot be attributed to basal sliding rates. Subglacial till acts as lubricating ‘fault gouge’ or ‘sawdust’, and must be removed for rapid subglacial bedrock erosion. Such erosion occurs especially where and when moulin-fed streams access the bed and are unconstrained by supercooling or other processes. Streams also may directly erode bedrock, likely with strong time-evolution. Erosion is primarily by quarrying, aided by strong fluctuations in the water system driven by variable surface melt and by subglacial earthquakes. Debris-bed friction significantly affects abrasion, quarrying and general glacier flow. Frost heave drives cirque headwall erosion as winter cold air enters bergschrunds, creating temperature gradients to drive water flow along premelted films to growing ice lenses that fracture rock, and the glacier removes the resulting blocks. Recent subglacial bedrock erosion and sediment flux are in many cases much higher than long-term averages. Over glacial cycles, evolution of glacial-valley form feeds back strongly on erosion and deposition. Most of this is poorly quantified, with parts open to argument. Glacial erosion and interactions are important to tectonic and volcanic processes as well as climate and biogeochemical fluxes, motivating vigorous research.

Deglaciation and its impact on permafrost and rock glacier evolution: New insight from two adjacent cirques in Austria

Glaciers and permafrost are strongly linked to each other in mid-latitude mountain regions particularly with polythermal glaciers. This linkage is not only climatically defined but also in terms of geomorphic and glaciological processes. We studied two adjacent cirques located in the Central Austria. We focussed on the deglaciation since the Little Ice Age (LIA) maximum (c.1850CE) and its relevance for permafrost and rock glacier evolution since then. One cirque is occupied by a glacier remnant whereas the second one is occupied by an active rock glacier which was partly overridden by a glacier during the LIA. We applied a multidisciplinary approach using field-based techniques including geoelectrics, geodetic measurements, and automatic monitoring as well as historic maps and photographs, remote sensing, and digital terrain analysis. Results indicate almost complete deglaciation by the end of the last millennium. Small-scale tongue-shaped landforms of complex origin formed during the last decades at finer-grained slope deposits below the cirque headwalls. Field evidences and geophysics results proved the existence of widespread sedimentary ice beneath a thin veneer of debris at these slopes. The variable thickness of the debris layer has a major impact on differential ablation and landform evolution in both cirques. The comparison of digital elevation models revealed clear mass losses at both cirques with low rates between 1954 and 2002 and significantly higher rates since then. The central and lower part of the rock glacier moves fast transporting sediments and ice downvalley. In contrast, the upper part of the rock glacier is characterised by low debris and ice input rates. Both effects cause a significant decoupling of the main rock glacier body from its nourishment area leading eventually to rock glacier starvation. This study demonstrates the importance of a decadal-scale and multidisciplinary research approach in determining the development of alpine landforms over both space and time.

Chronological and geomorphological investigation of fossil debris-covered glaciers in relation to deglaciation processes: A case study in the Sierra de La Demanda, northern Spain

In this study, fossil debris-covered glaciers are investigated and dated in the Sierra de la Demanda, northern Spain. They are located in glacial valleys of approximately 1 km in length, where several moraines represent distinct phases of the deglaciation period. Several boulders in the moraines and fossil debris-covered glaciers were selected for analysis of 10Be surface exposure dating. A minimum age of 17.8 ± 2.2 ka was obtained for the outermost moraine in the San Lorenzo cirque, and was attributed to the global Last Glacial Maximum (LGM) or earlier glacial stages, based on deglaciation dates determined in other mountain areas of northern Spain. The youngest moraines were dated to approximately 16.7 ± 1.4 ka, and hence correspond to the GS-2a stadial (Oldest Dryas). Given that the debris-covered glaciers fossilize intermediate moraines, it was deduced that they developed between the LGM and the Oldest Dryas, coinciding with a period of extensive deglaciation. During this deglaciation phase, the cirque headwalls likely discharged large quantities of boulders and blocks that covered the residual ice masses. The resulting debris-covered glaciers evolved slowly because the debris mantle preserved the ice core from rapid ablation, and consequently they remained active until the end of the Late Glacial or the beginning of the Holocene (for the San Lorenzo cirque) and the Holocene Thermal Maximum (for the Mencilla cirque). The north-facing part of the Mencilla cirque ensured longer preservation of the ice core.

Anomalous terrain at Dove Crags ‘cirqueform’ and Gasgale Gill asymmetric valley, English Lake District, attributed to large-scale rock slope failure of pre-LGM origins

An unusually bold, cirque-like cavity, above a small, apparently fluvial side valley rich in landscape anomalies, is evaluated as a large-scale ( c . 1 km 2 ) rock slope failure (RSF) with possible pre-Last Glacial Maximum (LGM) origins. Although the failed mass is much subdued, field evidence includes dry gullies and a tarnless hollow; a sharp flank scarp and a high cutbank toe suggest reactivation. Indirect support is provided by a pioneering (in Britain) schematic reconstruction of a whole-valley pre-RSF landscape, employing a ‘clone-stamping’ procedure, which resolves the main terrain anomalies. The RSF interpretation is satisfactorily tested by: (1) compatible proxy cavity and slipmass volumes of 35 and 25 million m 3 respectively, with the deficit attributed to trimming by an LGM valley glacier; (2) comparisons with other RSFs by area, cavity parameters, and a total extant volume (derived from an inferred balanced long section) of c . 50 million m 3 ; and (3) a proposed ‘zone of crush’ model that allows a large failed mass, c . 100–200 m thick, to descend incrementally by c . 300 m without disintegrating. A landscape paradox is identified whereby the valley now has an asymmetric, quasi-fluvial profile, with till covering the slope below the cirqueform cavity, yet is inferred to have had a pre-RSF glacial form and an LGM glacier. This is resolved by envisaging: (a) that a pre-LGM RSF created a cavity that was exploited by cirque glaciers on at least two occasions; (b) that the RSF was pared back by an LGM valley glacier; and (c) that it subsequently re-descended to the valley floor after the LGM. A pre-LGM origin for the RSF would account for the till-covered slipmass. It also allows time for excavation of the present cirque headwall by 50–100 m, and for the opposite valleyside to erode back by c . 50 m in response to a progressively displacing river and valley glacier. If this proposition is correct, Dove Crags would be one of the first substantial pre-LGM RSFs to be recognized in the glaciated mountains of Britain and Ireland, implying that others ought to be present. Supplementary material: A PDF illustrating Dove Crags and comparator sites, with additional discussion of regional context and cirque seeding by RSF. References in the text are given as (SI-42) etc. It is organised by theme, to view as a presentation, not in order of citation in the text and is available at http://www.geolsoc.org.uk/SUP18892

Glacier variations in the European Alps at the end of the last glaciation

The Last Glacial Maximum in the Alps lasted from approximately 30 to 19 ka. Glaciers reached out onto the forelands on both sides of the main Alpine chains, forming piedmont lobes in the north and filling the Italian amphitheatres to the south. Pullback of glaciers from their maximum extent was underway by 24 ka. Glaciers oscillated at stillstand and minor re-advance positions for several thousand years forming Last Glacial Maximum (LGM) stadial moraines. North and south of the Alps, the various stadials cannot yet be unequivocally matched. Glaciers had receded back within the mountain front by 19-18 ka. During the early Lateglacial phase of ice decay remnants of the once huge valley glaciers that fed the piedmont lobes downwasted and were likely calving into the extensive lakes that formed in the lower valley reaches. The first Alpine-wide glacier re-advance took place during the Gschnitz stadial, 17-16 ka, which was likely a response to Europe-wide cooling during Heinrich event 1. By the Bølling/Allerød interstadial much of the Alps were ice-free. Glaciers advanced repeatedly to an extent several kilometers from the cirque headwalls, during the Egesen stadial in response to the Younger Dryas cold period. Egesen stadial moraines, at some sites several sets of moraines, were constructed in valleys all across the Alps. 10Be exposure dates for Egesen stadial moraines are in the range 13.5 to 12 ka. Moraines located at an intermediate position between the Little Ice Age moraines and the Egesen moraines formed at the margins of glaciers that advanced during the closing phase of the Egesen stadial or during the earliest Holocene at 10.5 ka.

Late Pleistocene deglaciation in the upper Gállego Valley, central Pyrenees

Deglaciation processes in the upper Gállego Valley, central–southern Pyrenees, were studied using geomorphological mapping and 36Cl cosmogenic dating of moraine and rock glacier boulders, as well as polished bedrock. Although the precise position of the Gállego Glacier during the global last glacial maximum is not known, there is evidence that ice tongues retreated to the headwaters, which caused subdivision of the main glacier into a number of individual glaciers prior to 17ka. A range of ages (16 to 11ka) was found among three tributary valleys within the general trend of deglaciation. The retreat rate to cirque was estimated to be relatively rapid (approximately 5km per ka). The mapped glacial sedimentology and geomorphology appears to support the occurrence of multiple minor advances and retreats, or periods of stasis during the late deglaciation. Geomorphological and geological differences among the tributary valleys, and error estimates associated with the results obtained, prevented unambiguous correlations of the advances with the late Pleistocene cold periods. During the latter advances, small glaciers and rock glaciers developed close to the cirque headwalls, and co-occurred under the same climatic conditions. No evidence for Holocene re-advance was found for any of the three tributary valleys.

Maximum extent of Late Pleistocene glaciers and last deglaciation of La Cerdanya mountains, Southeastern Pyrenees

This paper examines glacial evolution in the La Pera and Malniu cirques, and Arànser, La Llosa and Duran valleys, in the Cerdanya massifs on the south-facing slopes of the eastern Pyrenees. A geomorphologic analysis and dating of moraine boulders, glacially polished bedrock and rock glacier blocks were carried out by means of cosmogenic 36Cl surface exposure dating. The maximum ice advance was contemporary with the Last Glacial Maximum at 23ka ago, and it was of greater or only slightly lesser magnitude than for previous Quaternary advances. The termini of glaciers remained close to maximum positions, with minor advances and retreats until 18–17ka when the glacial tongues disappeared from the valleys. Depending on the previous topography, these glaciers left behind a single polygenic moraine, in the case of confined valleys, or multiple moraines next to each other in the case of flat, more open areas. A final glacial advance is detected during the Oldest Dryas close to the cirque headwalls, and the glaciers finally disappeared during the Bølling interstadial. The glaciers were then replaced by rock glaciers, whose front immediately became inactive, although their activity continued near their source area until the early Holocene.

Glacial and periglacial buzzsaws: fitting mechanisms to metaphors

The buzzsaw hypothesis refers to the potential for glacial and periglacial processes to rapidly denude mountains at and above glacier Equilibrium Line Altitudes (ELAs), irrespective of uplift rates, rock type or pre-existing topography. Here the appropriateness of the buzzsaw metaphor is examined alongside questions of the links between glacial erosion and ELAs, and whether the glacial system can produce low-relief surfaces or limit summit heights. Plateau fragments in mountains on both active orogens and passive margins that have been cited as products of glacial and periglacial buzzsaw erosion instead generally represent dissected remnants of largely inherited, pre-glacial relief. Summit heights may correlate with ELAs but no causal link need be implied as summit erosion rates are low, cirque headwalls may not directly abut summits and, on passive margins, cirques are cut into pre-existing mountain topography. Any simple links between ELAs and glacial erosion break down on passive margins due to topographic forcing of ice-sheet growth, and to the km-scale vertical swaths through which ELAs have shifted through the Quaternary. Glaciers destroy rather than create low-relief rock surfaces through the innate tendency for ice flow to be faster, thicker and warmer along valleys. The glacial buzzsaw cuts down.

10Be exposure age chronology of the last glaciation in the Krkonoše Mountains, Central Europe

A new chronology of the last glaciation is established for the Krkonoše (Giant) Mountains, Central Europe, based on in-situ produced 10Be in moraine boulders. Exposure ages and Schmidt Hammer rebound values obtained for terminal moraines on the northern and southern flank of the mountains suggest that the oldest preserved moraines represent early phases of the Last Glacial Maximum (LGM). Large moraines at the outlet of the Snowy Cirques (Śnieżne Kotły) and in the middle part of the Úpa (Obří důl) trough were deposited around 21ka while a series of smaller moraines above the LGM deposits represent readvances that occurred no later than 18.1±0.6ka, 15.7±0.5ka, 13.5±0.5ka and 12.9±0.7ka. An exposure age of 13.8±0.4ka obtained for protalus ramparts at the foot of the Úpská jáma Cirque headwall indicates that glaciers advanced only in north- to east-facing cirques during the Lateglacial. The last glacier fluctuation was synchronous with the Younger Dryas cold event. The timing of local glacier advances during the last glacial episode correlates with the late Weichselian glacier phases in the Alps and in the Bavarian/Bohemian Forest.

Holocene environmental change resets lichen surface dates on Recess Peak glacial deposits in the Sierra Nevada, California

Development of an accurate chronology for glacial deposits in the Sierra Nevada has long been problematic given the lack of suitable organic material for radiocarbon dating. Lichenometry initially appeared promising as ages showed an increase from cirque headwalls to down-canyon moraines. However, while Recess Peak lichen age estimates range from 2 to 3ka, recent work shows these deposits to be at least 10ka older. Here, we present evidence for a late Holocene reset of Recess Peak lichen ages by significant post-depositional climate change. Following late-Pleistocene deposition of Recess Peak moraines, warming through the mid-Holocene allowed forests to advance into shallow basins eliminating local inverted tree lines. This produced a partial canopy where shading killed the original post-Pleistocene crustose lichen colonies. Late-Holocene cooling resulted in forest retreat from these basins as alpine tree line fell. Lichens then recolonized the re-exposed Recess Peak deposits. We conclude that while Recess Peak lichen ages are accurate to within the dating uncertainty of the technique, existing lichen ages actually date the timing of post-mid-Holocene cooling and recolonization, and not the original emplacement of these deposits. Thus, applications of Lichenometry should consider post-depositional environmental change when interpreting the meaning of these dates.

Periglacial weathering and headwall erosion in cirque glacier bergschrunds

Glaciers produce cirques by scouring their beds and sapping their headwalls, but evidence to constrain models of these processes has been elusive. We report a suite of environmental measurements from three cirque glacier bergschrunds, including the first temperature series recorded at depth throughout most of an annual cycle. Compared to the ambient air, the bergschrunds were colder in summer and warmer in winter. Freeze-thaw cycles were rare, and relatively stable subfreezing temperatures persisted from November until May. Using a model for rock fracturing driven by ice segregation, we demonstrate that favorable conditions for fracturing occur not only on the headwall above the glacier, but also within the bergschrund, where periglacial weathering and glacial transport can act together to drive cirque headwall retreat. A small (∼3 °C) year-round decrease in temperatures to conditions more typical of the Pleistocene would likely intensify the weathering process. Though so far ignored in all glacial landscape evolution models, the bergschrund likely plays an essential role in the sculpting of alpine landscapes.

The deglaciation of the Sierra Nevada (Southern Spain)

Cosmogenic 36Cl surface exposure dating was carried out on 19 samples from glacially polished bedrock, rock glaciers and moraines of four glaciated valleys with different orientations in the Sierra Nevada (Spain) around Pico del Veleta (37° 3′N 3° 21′W, 3398m asl). Remains of moraines older than the global Last Glacial Maximum exist, but their poor preservation makes them an unreliable subject for surface exposure dating. Results show that the last local glacial maximum advance took place earlier than the global Last Glacial Maximum, but within the MIS 2, although additional dating is needed to confirm this. Exposure ages from bedrock steps show a general glacier retreat in progress by 15–14ka. Shortly afterwards an extensive system of rock glaciers formed at the base of the valley headwalls, with the most recent rock glacier lasting until 7ka ago. From their location, morphology and analysis of other proxies, we interpret their origin as related to the deglaciation process and marked instability of cirque headwalls, rather than to extreme periglacial conditions. A recent similar occurrence was the transformation of the Little Ice Age glacier on the north face of the Pico del Veleta into a rock glacier under temperate high mountain climatic conditions.

Cirque overdeepening and their relationship to morphometry

Shape and size characteristics of cirques and their changes and mutual relationships include not only spatial and descriptive aspects but also genetic attributes. The goals of this article were to define an application of k-curve (sensu Haynes, 1968) that describes the extent of cirque overdeepening and to identify k-curve equation input variables. The k-values depend on the profile location, and hence, a clear definition of the cirque profile position is necessary. Therefore, ideal profiles were delineated through the steepest part of the cirque headwall ( k-steepest, k s) or through a point on the cirque crest that had a maximal elevation ( k-highest, k h). Knowledge of the cirque headwall height allows for the mathematical (theoretical) calculation of cirque overdeepening (i.e., the maximum depth of the cirque floor, sedimentary filling of the cirque floor). Further, the goal of this study was to analyse and compare cirque classification methods according to the extent of overdeepening and standard morphometric characteristics. As a training data set, 27 example cirques of the Bohemian Massif (Giant Mts., Šumava Mts., and Hrubý Jeseník Mts.) and a total of 12 morphometric characteristics were used in the analyses. Cirques were classified into two groups using cluster analysis based on the extent of overdeepening ( k-values: k h and k s). A mutual relationship between cluster analysis classifications and classifications based on morphometric characteristics ( L/ W, H, and 3D/ 2D) was determined using general discriminant analysis. The classification of cirques based on other characteristics corresponded closely (in total, 81% for the first group and the cross-validation group) with cirque classifications based on the degree of overdeepening ( k-value).

Use of 10Be exposure ages and Schmidt hammer data for correlation of moraines in the Krkonoše Mountains, Poland/Czech Republic

Exposure ages and relative-age data are presented from eight sites in the Łomnica and Łomniczka valleys to provide essential information for reconstructing local glaciation chronology. A combination of 10Be exposure ages and Schmidt hammer data obtained for moraines indicate relatively short period of glacier accumulation lasting from 17.0 0.4 ka to 13.6 0.9 ka. Exposure age of 8.4 0.3 ka measured on the lowermost section of the Łomnica cirque headwall further confirms the view of glacier preservation in favourable sites until the beginning of the Holocene. A comparison of the obtained chronological data with timing of mountain glaciation in the nearby Upa Valley is used to propose the first correlative model of Late Quaternary glaciation in the eastern part of the Krkonose Mountains. The correlation implies that the lowermost preserved moraines originated during the local maximum of the last glaciation whereas recessional moraines were deposited until the Lateglacial period. A subsequent melting of glaciers terminated at the beginning of the Holocene. The implications of the model are discussed and further investigations are suggested to extend its validity to the whole mountain area.

Cirques, peaks, and precipitation patterns in the Swiss Alps: Connections among climate, glacial erosion, and topography

Glacial erosion, a process influenced by climate, has been implicated in limiting the relief of mountain ranges. However, climate itself is sensitive to large-scale topography, suggesting that climate, topography, and glacial processes form a coupled system. Large spatial gradients in precipitation exist in the southern Swiss Alps, allowing us to study this coupling in a region of climate variability. More than 500 cirques were identified, and neighboring peaks were found to co-vary in elevation with cirque floors ( R 2 0.64). Cirque headwall relief does not vary with precipitation or cirque floor altitude. These relationships confirm the hypothesis that cirque formation restricts peak altitudes via slope processes that limit the relief of cirque headwalls. We compared the position of the regional equilibrium line altitude (ELA) estimated from modern climate to a surface defined by the cirque floors. The modern ELA and cirque floor surfaces are similar in shape, illustrating the impact of spatial variability in precipitation on glacial processes and topography. Precipitation variability in this region is partially dictated by large-scale topography. Therefore, precipitation patterns, glacial process domains, and topography must evolve together.

Lichens and bryophytes of the alpine and subalpine zones on Katahdin, Maine, II: Lichens

A total of 293 different lichens (and lichenicolous fungi) were found above an elevation of 884 m on Katahdin during fieldwork from 2000 to 2004 and from study of previously collected herbarium specimens. Thirteen of these are new to North America, 39 additional lichens are new to northeastern United States and 26 additional ones are new to Maine; three additional taxa are almost certainly new to science but are not formally described in this study. We identified eight distinct lichen habitats on Katahdin: (1) A1, exposed alpine tundra and fellfields; (2) A2, high elevation alpine sites protected from northwest winds; (3) A3, east-facing alpine late-snow areas; (4) A4, alpine krummholz; (5) A5, cirque headwalls facing east and north; (6) A6, cirque headwall facing southeast; (7) B1, exposed low-altitude tundra, talus and krummholz; and (8) B2, subalpine forests. Each habitat except A4 had numerous lichens (10–56 taxa) found in none of the others, and each differed statistically from two to six other habitats by a quantitative measure of the Arctic-boreal-temperate (ABT) distribution of its lichens. Habitat A3 had, on average, the most Arctic lichens, followed by A1, A2, A5, B1, A6 and A4, with B2 having the fewest Arctic lichens and the most with a temperate distribution. Lichen ABT values for the eight habitats are well predicted (R2 = 96.9%) by a multiple regression equation incorporating three independent variables: average elevation above sea level of the lichens collected in a habitat class; an estimate of solar gain for each of the eight habitats; and the presence or absence of trees (including krummholz). We compared our results with those for Mt. Albert, Gaspé, Quebec, the only other comprehensive study of alpine lichens in northeastern North America, for such parameters as species presence/absence, substrata and ABT value. We suggest that future studies looking into effects of global warming or increases or decreases in air pollution will be facilitated by the study of certain habitats rather than the entire alpine and subalpine Katahdin region.

Development of tongue-shaped and multilobate rock glaciers in alpine environments – Interpretations from ground penetrating radar surveys

Abstract Rock glaciers occur as lobate or tongue-shaped landforms composed of mixtures of poorly sorted, angular to blocky rock debris and ice. These landforms serve as primary sinks for ice and water storage in mountainous areas and represent transitional forms in the debris transport system, accounting for ~ 60% of all mass transport in some alpine regions. Observations of active (flowing) alpine rock glaciers indicate a common association between the debris that originates from cirque headwalls and the depositional lobes that comprise them. The delivery of this debris to the rock glacier is regulated primarily by the rate of headwall erosion and the point of origin of debris along the headwall. These factors control the relative movement of individual depositional lobes as well as the overall rate of propagation of a rock glacier. In recent geophysical studies, a number of alpine rock glaciers on Prins Karls Forland and Nordenskioldland, Svalbard, Norway, and the San Juan Mountains of southwest Colorado, USA, have been imaged using ground penetrating radar (GPR) to determine if a relationship exists between the internal structure and surface morphology. Results indicate that the overall morphologic expression of alpine rock glaciers is related to lobate deposition during catastrophic episodes of rockfall that originated from associated cirque headwalls. Longitudinal GPR profiles from alpine rock glaciers examined in this study suggests that the difference in gross morphology between the lobate and tongue-shaped rock glaciers can be attributed primarily (but not exclusively) to cirque geometry, frequency and locations of debris discharge within the cirque, and the trend and magnitude of valley gradient in relation to cirque orientation. Collectively, these factors determine the manner in which high magnitude debris discharges, which seem to be the primary mechanism of formation, accumulate to form these rock glaciers.